No way to specify client context for local invoke

Description

I am running a local service with start-lambda and interacting with it from the Node SDK. It appears that the function ClientContext is undefined despite sending it correctly with the SDK.

There is also no way to specify ClientContext with the local invoke command.

Steps to reproduce

1. Write a simple lambda function

exports.processEvents = function (event, context, callback) {
    console.log("event", event)
    console.log("context", context)
})

1. Start local lambda service

$ sam local start-lambda
2019-05-13 17:48:27 Found credentials in shared credentials file: ~/.aws/credentials
2019-05-13 17:48:27 Starting the Local Lambda Service. You can now invoke your Lambda Functions defined in your template through the endpoint.
2019-05-13 17:48:27  * Running on http://127.0.0.1:3001/ (Press CTRL+C to quit)

1. Invoke function with AWS SDK

#!/usr/bin/env node
var fs = require('fs');

var AWS = require('aws-sdk');
var Lambda = new AWS.Lambda({
    endpoint: 'http://127.0.0.1:3001/',
});

Lambda.invoke({
    ClientContext: new Buffer('{"apiKey": "abcd1234"}').toString('base64'),
    FunctionName: "ProcessEventsFunction",
    Payload: fs.readFileSync("event.json"),
}, function(err, data) {
    console.log("err", err)
    console.log("data", data)
})

Observed result

See that context.clientContext is undefined

2019-05-13 17:52:30 Invoking index.processEvents (nodejs10.x)
Fetching lambci/lambda:nodejs10.x Docker container image......
2019-05-13 17:52:31 Mounting /Users/noah/dev/partnerapp/lambda as /var/task:ro,delegated inside runtime container
START RequestId: 52fdfc07-2182-154f-163f-5f0f9a621d72 Version: $LATEST
  event: 'Registered',3Z        52fdfc07-2182-154f-163f-5f0f9a621d72 

INFO    event { type: 'track',
  userId: 'test-user-23js8',
  timestamp: '2019-04-08T01:19:38.931Z',
  email: '[e-mail]',
  properties: { plan: 'Pro Annual', accountType: 'Facebook' } }
  succeed: [Function],4Z        52fdfc07-2182-154f-163f-5f0f9a621d72

INFO    context { callbackWaitsForEmptyEventLoop: [Getter/Setter],
  fail: [Function],
  done: [Function],
  functionVersion: '$LATEST',
  functionName: 'test',
  memoryLimitInMB: '128',
  logGroupName: '/aws/lambda/test',
  logStreamName: '2019/05/14/[$LATEST]ea74a58b6601ed93e4664022f15bf42d',
  clientContext: undefined,
  identity: undefined,
  invokedFunctionArn: 'arn:aws:lambda:us-east-1:1178758407:function:test',
  awsRequestId: '52fdfc07-2182-154f-163f-5f0f9a621d72',
  getRemainingTimeInMillis: [Function: getRemainingTimeInMillis] }
END RequestId: 52fdfc07-2182-154f-163f-5f0f9a621d72
REPORT RequestId: 52fdfc07-2182-154f-163f-5f0f9a621d72      Duration: 19.25 ms      Billed Duration: 100 ms Memory Size: 128 MB     Max Memory Used: 46 MB  

Expected result

Expect the ClientContext value sent with the Invoke API call would be available in my function.

Additional environment details (Ex: Windows, Mac, Amazon Linux etc)

1. OS: OS X
2. sam --version: SAM CLI, version 0.16.0

[2matzzz]

+1

[rodmaz]

+1 This is definitely an important missing capability in sam.